Field of the Invention
Embodiments of the invention relate to methods for assembling turbomachine blades on a turbomachine rotor, in particular blades for an axial turbomachine, such as a gas turbine, an axial compressor, or a steam turbine. The disclosed subject matter also relates to a turbomachine rotor.
Description of the Related Art
A turbomachine drum rotor usually comprises a drum with a blade-retaining groove circumferentially developing around the drum and having a generally T-shaped cross section. The blades comprise each an airfoil portion and a root portion, which is generally T-shaped and intended for retention in the blade-retaining groove of the rotor.
The blades are constrained to the rotor by introducing the root portion in the blade-retaining groove and thereafter twisting the blade about a radial axis, to engage the root portion in the undercut formed by the T-shaped blade-retaining groove.
The number of blades must be sufficient to form a complete annular blade arrangement and are tangentially forced one against the other to resist pressure and vibrations. Several solutions have been developed to introduce the blades in the T-shaped groove and finally force them tangentially one against the other.
In some known turbine rotor arrangements, in order to assemble a complete ring of blades around the rotor, the last blade to be introduced has a root portion which is not T-shaped and which is introduced in an insert space which has, with respect to the width of the T-shaped blade-retaining groove, a larger dimension in the axial direction, i.e. in a direction parallel to the axis of rotation of the rotor. The last blade is retained by locking it with two insertion pieces introduced in the insert space, with the aid of radial screws. When the last blade is introduced and locked, a complete blade ring is formed and the blades are tangentially forced one against the other. U.S. Pat. No. 7,901,187 discloses this kind of construction.
FIG. 23 schematically illustrates a portion of a turbine rotor and relevant blades, showing in particular the last blade which has been mounted on the rotor. The rotor is indicated with reference number 100. Blades 102 are mounted around the rotor and retained in an undercut blade-retaining groove, e.g. having a generally T-shaped cross section, and extending circumferentially around the rotor. Each blade except the last one has a T-shaped root portion (not shown) engaging the undercut groove. The blades 102 are introduced into the blade-retaining groove in correspondence of an insert space shown at 103. The last blade 105 is introduced in the insert space 103 after insertion therein of two opposed insert pieces 107. The insert pieces 107 and the last blade 105 are locked on the rotor or drum 100 by means of screws 109, 111.
This known mounting system has some drawbacks, including a reduced efficiency in the retention of the last blade 105. The latter is radially retained against the centrifugal forces, which are generated during rotation of the rotor, by means of the screws 109, 111. In order to obtain a sufficient radial retention effect, the screws must deeply engage into the rotor. This results in stress concentration, especially in turbomachines subject to high operating temperatures, such as those arising in steam turbines.
U.S. Pat. No. 7,168,919 describes a further known arrangement for mounting and tangentially locking the blades on a rotor drum. In this known arrangement, each blade has a root with opposite raised portions extending in the axial direction of the root. The blades are introduced in the T-shaped groove in a radially staggered arrangement, so that the respective raised portions are initially radially staggered. Finally the blades are displaced radially outwardly so that the raised portions of all the blades are in radial alignment thus eliminating the clearance between adjacent blades and forcing the blades one against the other in the tangential direction. Machining of the blades is very complex and in the assembling process it is very difficult to control and adjust the final tangential interference.
In other known arrangements, shims are forcedly introduced between roots of adjacent blades, to generate tangential interference between the blades and force them one against the other in tangential direction. The shims are locked by means of screws. Also this arrangement proved not to be satisfactory since it requires critical machining at assembly. In addition the shims must be thick to be forcedly introduced and to host the retaining screws. This requires blades of uneven root pitch, so that the blade row cannot be optimized from the point of view of stress resistance.
There is therefore a need for a more efficient system of mounting the blades on a turbomachine rotor and especially a more efficient way of inserting the last blade and closing the whole blade ring.